1. Field of the Invention
The present invention relates to a polishing apparatus.
2. Technical Field of the Invention
In recent years, polishing apparatuses have been used to polish surfaces of substrates such as semiconductor wafers. While rotating a polishing table to which a polishing pad (for example, a nonwoven cloth or a urethane rein) for polishing the substrate is attached, the polishing apparatus feeds a polishing and grinding liquid (slurry) onto the polishing pad. The polishing apparatus also holds the substrate by a top ring and pressurizes a back surface of the substrate to press the substrate against the polishing pad, thus polishing a front surface of the substrate.
On the other hand, when polishing of the substrate ends, the polishing apparatus uses the top ring to switch from pressurization to sucking of the back surface of the substrate. The polishing apparatus then moves the top ring upward with the substrate sucked thereto, thus lifting the substrate (elevating the substrate in a direction away from the polishing pad). This step of lifting the substrate may be carried out after the substrate is cleaned by feeding a cleaning solution such as pure water onto the polishing pad; the cleaning of the substrate follows the step of polishing the substrate.
During the step of lifting the substrate, the substrate may be left on the polishing pad and fail to be lifted. That is, after the substrate is polished, the polishing and grinding liquid or pure water is present on the polishing pad. Thus, a vacuum (negative pressure) region may be formed between the substrate and the polishing pad. In this case, under a vacuum pressure, the top ring and the substrate pull each other and the substrate and the polishing pad pull each other. If the vacuum pressure between the substrate and the polishing pad is higher than the vacuum pressure between the top ring and the substrate, the substrate may be sucked to the polishing pad and the top ring may elevate with the substrate left on the polishing pad.
In this case, as is well known, a conventional technique allows the polished substrate to be sucked to the top ring, and before lifting the substrate, slidably moves the substrate in a direction along a polishing surface of the polishing pad (a lateral direction) so that the substrate partly projects out from the polishing pad. This reduces the suction force exerted between the substrate and the polishing pad to facilitate lifting of the substrate.
As is also well known, another conventional technique increases a duration for which the substrate is sucked to the top ring to improve the suction force between the top ring and the substrate. This facilitates lifting of the substrate.
However, the conventional technique fails to take into account improvement of the capability of lifting the substrate with the performance of substrate polishing maintained.
That is, with the conventional technique of moving a part of the substrate out from the polishing pad before lifting the substrate, the substrate may fall down from the polishing pad and be damaged. Furthermore, when moved out from the polishing pad, the substrate may be scratched (flawed).
Additionally, with the conventional technique of increasing the duration for which the substrate is sucked to the top ring, a speed at which the substrate is lifted decreases to reduce the throughput of substrate polishing. This is not preferable for the polishing apparatus, which is required to efficiently process a large number of substrates.
Thus, an object of the present invention is to improve the capability of lifting the substrate with the polishing performance of the substrate maintained.